FIG. 1 shows a typical prior art differential pair used as a driver for optical network laser diodes. The circuit of FIG. 1 includes transistors Q1-Q6; resistors R0, R1, and R2; current sources I3, I4, I12, and I56; input nodes INP and INM; base nodes BP and BM; tail current node T; supply node Vcc; and output nodes OUTP and OUTM. The differential pair formed by transistors Q5 and Q6 provides a large driver current for a laser diode connected at either output node OUTM or output node OUTP. In order to switch the high currents necessary for illuminating laser diodes, the differential pair is driven by emitter follower buffer and level shift devices Q3 and Q4. An amplifier stage formed by transistors Q1 and Q2 accepts low power inputs at input nodes INP and INM. Additional level shifting is provided by resistor R0.
When switching currents in a differential pair, an overshoot typically occurs in the output where current is being switched on. (See H. M. Rein, et al., “A Versatile Si-Bipolar Driver Circuit with High Output Voltage Swing”, IEEE Journal of Solid State Circuits, September 1994, page 1014.) For example, assume in FIG. 1 that transistor Q5 is initially off and that the tail current I56 is flowing through transistor Q6 to the output node OUTM. To switch the current I56 to output node OUTP, input node INM is raised and input node INP is lowered. This causes a negative going transient at the base of transistor Q6 (node BM). Since transistor Q6 had been conducting all of current I56, its cπ (the effective small-signal capacitance between base and emitter) is quite large. The cπ of transistor Q5, however, is quite small since transistor Q5 was initially off, giving an inherent asymmetry. The negative going transition at node BM needed to turn transistor Q6 off will couple through the large cπ capacitance of transistor Q6 and will drive the tail node T negative, and extract a large charge out of the emitter of transistor Q5. This current transient will appear in the collector of transistor Q5 as an overshoot in addition to the tail current I56 that is being diverted to transistor Q5. The plot of FIG. 2 shows the simulated transistor Q5 collector current waveform, and the overshoot can clearly be seen on top of the switched current of 10 mA.
The current overshoot in FIG. 2 is not acceptable in an optical network laser, as well as in many other applications where high speed differential current switching is required, such as high speed digital to analog converters. Previous circuit solutions to this problem have resulted in a loss of switching speed for the differential pair, or other drawbacks.